Many electronic applications include an oscillator circuit that converts a substantially constant current or voltage input signal to a periodic output signal having a substantially predetermined frequency. Certain oscillator circuits, for example, provide a substantially sinusoidal signal. These circuits, typically, employ a tuned element such as a crystal oscillator that generates a sinusoidal frequency in response to a voltage signal. Other oscillator circuits, such as ring oscillators, provide digital pulse signals having a substantially predetermined frequency. In certain applications, it is desired to vary the frequency of the output signal of an oscillator as a function of a control signal. For example, phase-locked loops (PLL's) include an oscillator that generates an output signal with a frequency, capable of being varied substantially in accordance with a control signal. In many instances, the frequency of this oscillator may be varied over a certain frequency range by a control voltage or current signal. These types of oscillators are referred to in this context as voltage or current-controlled oscillators.
Conventional ring oscillators are nonlinear in frequency. As the input control current signal to these ring oscillators varies, the oscillation frequency of the circuit does not respond linearly. However, in some applications, it is desirable to have a ring oscillator that is substantially linear in frequency. Therefore, a need exists for a ring oscillator that has a substantially linear frequency/current characteristic.